What a lovely day.

I think this might explain it.

From http://www.nuc.berkeley.edu/node/6239

Radon progeny in precipitation
Submitted by bandstra on Thu, 2012-03-01 12:25.

Hi, thanks for the question. Are you referring to my writeup on a measurement of radioactivity of a swipe of a metal surface after it rained? The writeup can be found at this link, and I will repeat some of its content here.

Rain is known to "scrub" the decay products of radon gas out of the atmosphere and deposit them on the ground. So it is not unusual to find measurable radioactivity in rain.

The absolute amount of the radioactivity is not helpful for distinguishing between different possible sources. For example, I measured only 45 counts per minute above background for my rain swipe, but that number will depend on various things: the detector I used, how large an area I wiped down, the concentration of radon decay products that had been in the air, etc.

What really matters, as BC alluded to, is the decay time of the radioactivity. The decay time can be ascertained very quickly, since the total activity of the decay products of radon gas should decay away with a half-life of about 30 minutes. This is due to the betas and gammas from Lead-214 (27 minute half-life) and Bismuth-214 (20 minute half-life). This half-life is far too short to be due to material transported from Japan.

Here is the decay curve of the rain activity I measured. I took 10-minute counts for about 3 hours:


As for snow, there is a nice blog post on Safecast.org where they measure snow in Japan with a Geiger counter and conclude that the radioactivity is due to radon decay products. They say that they measured rates "up to 10 times background," but the activity decayed away to background in "around 25–30 minutes."

You might also be interested in a measurement someone made of an air filter from their home — just like rain, air filters also collect radon decay products from the air. Initially he saw about 1000 counts per minute above background. The thread is here, and his data was also fit very well by a radon progeny decay model:


Please let me know if this explanation makes sense.

Mark [BRAWM Team Member]

There was also snowfall in Pennsylvania from steam from a nuclear power plant. I doubt it's radioactive, though. Or else we really are living in the post-apocalyptic 21st century. http://www.washingtonpost.com/blogs/capital-weather-gang/post/nucl ear-power-plant-produces-snow-in-southwest-pa/2013/01/23/1f880d54- 656c-11e2-85f5-a8a9228e55e7_blog.html

Just use a RadAway.